KIST researchers overturn existing theory, "Controlling the spread of oil in the subsurface, hydrophilic materials are more effective"
Simulate groundwater environments to see more oil spills in hydrophobic media
image: Schematic of a water-oil immiscible alternative microfluidic experiment conducted under constant differential pressure. The microfluidic chip used is a uniform type with a constant pore structure and a random rock type that simulates the actual underground environment.
Credit: Korea Institute of Science and Technology(KIST)
A common method for separating oil from water has been to use hydrophobic materials that adsorb oil. What the researchers found through close observation was the opposite of what they had expected. In conditions of flowing through a porous medium, such as groundwater, they found that hydrophilic surfaces - those that bind easily with water molecules - hold onto more oil.
Dr. Seunghak Lee, Jaeshik Chung, and Sang Hyun Kim of the Water Resources Cycle Research Center at the Korea Institute of Science and Technology (KIST) observed how oil and water interact in porous media under various conditions using a "microfluidic system" that allows precise observation of microscopic fluid flows. In particular, they conducted experiments under constant pressure differential conditions similar to real groundwater flow, and found that oil easily escaped from hydrophobic surfaces, while more oil was retained on hydrophilic surfaces. These observations were verified with a immiscible displacement analytical model.
In hydrophobic materials, the contact angle at the interface where water repels oil is larger than in hydrophilic media. This reduces the capillary pressure drop at the interface, but increases the pressure difference due to fluid viscosity, which in turn increases the velocity of the fluid in the pores. This accelerated flow of water in the pores causes more oil to spill out. In hydrophilic materials, on the other hand, the oil is not pushed out as well by the relatively low flow velocity in the pores under the same pressure conditions, resulting in a large amount of oil remaining.
This study goes beyond simple fluid behavior analysis and provides a new interpretive framework for the migration and settling of contaminants in groundwater. The results of this study are expected to contribute to the effective design and operation of pollution prevention facilities such as Permeable Reactive Barrier (PRB) to control oil pollution in groundwater, which is common at military bases and gas station sites.
"Groundwater remediation is not just a matter of materials science, but a representative multiphysics phenomenon that involves a complex interplay of fluid flow and interfacial reactions," said Dr. Jaeshik Chung, KIST. "This research can be applied not only to groundwater remediation, but also to various immiscible displacement processes in porous media, such as enhanced oil recovery (EOR) and carbon capture and storage (CCS)."
"This achievement shows that underground fluid flow can behave completely differently from existing scientific theories under certain conditions," said Dr. Seunghak Lee of KIST, adding, "This research lays the scientific foundation for more precise control of the underground environment."
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KIST was established in 1966 as the first government-funded research institute in Korea. KIST now strives to solve national and social challenges and secure growth engines through leading and innovative research. For more information, please visit KIST’s website at https://www.kist.re.kr/eng/index.do
This research was supported by the Ministry of Science and ICT (Minister Bae Kyung-hoon) and the Ministry of Environment (Minister Kim Sung-hwan) through the KIST Institutional Program and the Ground Pollution Hazard Management Technology Development Project (RS-2021-KE002011). The results of this research were published in the latest issue of the international journal "npj Clean Water" (IF 10.5, top 1.2% in JCR water resources).